The invention relates to temperature sensing devices, and more particularly to such devices using the temperature sensitive properties of transistors.
A variety of temperature measuring devices are well known in the art. Standard devices, such as thermocouples, thermistors, or RTDs, all have limitations which prevent them from being easily or widely utilized. Thermocouples require accurate cold junction compensation, some form of linearization, and produce a low level voltage output which is subject to electrical noise interference. Resistance thermometers and thermistors have a non-linear characteristic which requires careful compensation and a wide dynamic electrical range. In addition, making a good resistance measurement demands an accurate voltage source, low level precision current sensing, and careful lead compensation. These devices also require strict attention to lead wire material connections. Initial calibration on all of these devices is also a problem, especially when field replacement is necessary.
Conventional integrated circuit temperature transducers are based upon the capability of a transistor to generate a base-to-emitter difference voltage (.DELTA.V.sub.be) proportional to absolute temperature, accurate over a wide temperature range. Previous attempts to use this property have employed means for amplifying and buffering the voltage signal and providing necessary support circuitry, such as a voltage regulator, on the same integrated circuit chip.
One particular approach to using the linear .DELTA.V.sub.be versus temperature property of a transistor to form a temperature sensing device is described in U.S. Pat. No. 3,940,760 to Brokaw and U.S. Pat. No. 4,123,698 to Timko et al. A two terminal temperature transducer 10 illustrated in FIG. 1 generates an output current I.sub.T varying with absolute temperature by means of first and second transistors Q1 and Q2 operated at a constant ratio of emitter current densities, and having their bases interconnected and the difference between their respective V.sub.be impressed across a resistor R. In the transistors Q1 and Q2, the equation for emitter current density is: ##EQU1## where J.sub.s is the emitter saturation current density, q is the charge in coulombs of an electron, K is Boltzman's constant, and T is absolute temperature. In practice, the (-1) term is very small and is usually neglected.
For two transistors at current densities J.sub.e1 and J.sub.e2, the respective base-to-emitter voltages are: ##EQU2##
The difference between the base-to-emitter voltages is then given by: ##EQU3##
For .DELTA.V.sub.be to be proportional to absolute temperature, the logarithmic term must be constant. Thus, if J.sub.e1 /J.sub.e2 is a constant r, not equal to 1, then ##EQU4##
In the transducer 10 of FIG. 1, a constant ratio of emitter current densities is achieved by providing the first and second transistors Q1 and Q2 with emitter conductive areas of different sizes, and by using additional transistors Q3 and Q4 connected to the collectors of the transistors Q1 and Q2, together with diode connections across transistors Q2 and Q3, in order to supply currents through the transistors Q1 and Q2. Assuming that the transistor collector currents are dependent only on V.sub.be, and neglecting base currents, then equal currents I.sub.c1 =I.sub.c2 are forced through transistors Q1 and Q2. Assuming the emitter conductive areas of transistors Q1 and Q2 are in a ratio r, the ratio of emitter current densities also will be r, and the difference voltage .DELTA.V.sub.be is directly proportional to absolute temperature. The voltage .DELTA.V.sub.be appears across resistor R and determines the level of current flowing through transistor Q1. The output current I.sub.T drawn by both sides of the circuit is ##EQU5##
If the resistor R has a zero temperature coefficient, then I.sub.T is also directly proportional to absolute temperature, and appropriate selection of the emitter ratio r and resistance R will provide an output current accurately related to temperature with a predetermined constant of proportionality, useful for absolute temperature sensing purposes.
Unfortunately, while the above described transducer provides an output current which is proportional to absolute temperature, in practice the device is generally specified only for operation to -55.degree. C. or 218.degree. K. and is useful at somewhat lower temperatures. In order to measure a small temperature range using the transducer 10, it is necessary to suppress at least the first 218.degree. K. worth of its output signal, or more typically, the first 273.degree. K. in order to refer a conditioned output signal to 0.degree. C. This suppression is an inconvenience and a possible source of error with a degradation of signal-to-noise ratio. The zero suppression involves an additional reference signal which must be temperature insensitive, and any error in this reference reflects as an error in the conditioned output.
Accordingly, it is desirable to provide a two terminal temperature transducer which has a zero output occurring closer to its normal operating temperature range. Such an arrangement permits the transducer to operate with greater sensitivity since the effect of self heating by the operating current is smaller.
It is therefore an object of the present invention to provide an integrated circuit two terminal temperature transducer which measures temperature and indicates its output by modulating its operating current linearly proportional to temperature referred to some zero value above 0.degree. K. and close to its intended range of operation.